1. Field of the Invention
The present invention relates to an optical transmission system, and more particularly to a system for transmitting an optical signal from a transmitter to a receiver through a multi-mode fiber.
2. Description of the Background Art
The development in technologies in recent years has produced optical fibers which satisfy broadband requirements as well as low loss requirements. As a result, optical fibers are being introduced in the backbone systems for interconnecting exchange systems on a network (e.g., the Internet).Optical fibers are considered promising for future applications in access systems for interconnecting exchanges with households, and also applications in home networks.
Optical fibers can be generally classified in two types based on their characteristics: single mode fibers (hereinafter referred to as “SMFs ”) and multi-mode fibers (hereinafter referred to as “MMFs ”).In a SMF, both the core and the cladding are made of silica (SiO2). A SMF has a core diameter as small as about 10 μm. Furthermore, a SMF features a broad transmission bandwidth because it only allows a particular mode to be propagated therethrough. Therefore, SMFs have mainly enjoyed developments for long-distance and broadband transmission purposes in the backbone systems, and have gained wide prevalence there.
On the other hand, a MMF has a core diameter of 50μm to 1 mm, which is greater than the core diameter of a SMF. MMFs can be classified in several types based on the materials of the core and cladding. MMFs whose core and cladding are both made of silica are called GOFs (Glass Optical Fibers). MMFs whose core is made of silica, and whose cladding is made of a polymer, are called PCFs (Polymer Clad Fibers). MMFs whose core and cladding are both plastic are called POFs (Plastic Optical Fibers).
AMMF has a plurality of propagation modes (i.e., optical paths). FIG. 12 is a schematic diagram illustrating a plurality of propagation modes. In FIG. 12, a MMF 73 has a core 71 and a cladding 72. The entirety of light travels through the core 71 while being repeatedly reflected at the boundary Fbd between the core 71 and the cladding 72 (TIR: Total Internal Reflection). Therefore, modes which are closer to being parallel to the boundary Fbd will travel longer distances over the fiber axis between one reflection and the next reflection. Such modes (denoted by dot-dash lines) are referred to as lower-order modes (MLO). On the other hand, modes which travel shorter distances over the fiber axis between one reflection and the next reflection (denoted by double-dot-dash lines) are referred to as higher-order modes (MHI) A higher-order mode MHI constitutes a relatively large angle with respect to the fiber axis. Therefore, given a fixed length of the MMF 73, a higher-order mode MHI will experience a larger number of reflections at the boundary Fbd than a lower-order mode MLO, thus presenting an optical path which is different from that of the lower-order mode MLO (“optical path difference”). Due to optical path differences, different modes require different amounts of time to travel from an input plane to an output plane of the MMF 73.
An optical signal is transmitted through an optical fiber in the form of a pulse sequence. Since each mode in the optical signal has its own inherent propagation speed, a pulse sequence which is contained in a lower-order mode MLO (which has a relatively short propagation time) and the same pulse sequence which is contained in a higher-order mode MHI (which has a relatively long propagation time) will arrive at the receiving end at different times, although directed to the same information. As a result, the receiving end of the information may not be able to correctly receive the signal. This phenomenon, known as mode dispersion, is a factor which considerably constrains the transmission bandwidth of a MMF as compared to that of a SMF.
A transmission bandwidth of an optical fiber is usually represented as a product of a data rate for optical signals transmitted therethrough and a transmission distance (e.g., Mbps×km). The transmission distance must be decreased as the data rate is increased. In order to increase the transmission distance, the data rate must be lowered. The influence of mode dispersion also becomes more significant as the data rate is increased, or as the transmission distance is increased. Therefore, conventional optical transmission systems employing MMFs have a problem in that the transmission distance must be compromised in order to obtain a necessary data rate.
However, MMFs are less expensive than SMFs. Therefore, on the bare comparison, an optical transmission system employing MMFs should be able to be constructed inexpensively as compared to a system employing SMFs. Moreover, since the core diameter of a MMF is greater than that of a SMF, it is relatively easy to align the axes of two MMFs with each other. This helps relaxing the mounting precision of a connector for interconnecting MMFs. Thus, MMFs can greatly contribute to the construction of a low-cost optical transmission system. Therefore, MMFs are preferred for optical transmission over a distance which is short enough for the mode dispersion effects to be negligible.
In order to take advantage of the aforementioned features of MMFs, a number of techniques for reducing the influence of mode dispersion in MMFs and for improving the transmission bandwidth of an optical transmission system have been proposed. With reference to FIGS. 13 and 14, a technique disclosed in Japanese Patent Laid-Open Publication No. 10-227935 will be described. FIG. 13 is a block diagram illustrating the overall structure of a conventional optical transmission system Scv. As shown in FIG. 13 , the optical transmission system Scv includes a light source 82 having a lens 81, a MMF 83, a mode separator 84, and a receiver 85. FIG. 14 is a schematic diagram illustrating the optical coupling between the lens 81 and the MMF 83 shown in FIG. 13. As shown in FIG. 14, the lens 81 and the MMF 83 are disposed so as to attain a maximum coupling efficiency. Specifically, the MMF 83 is affixed in such a manner that an optical axis Alz (denoted by a dot-dash line) of the lens 81 and a fiber axis Afr (denoted by a double-dot-dash line) of the MMF 83 are on a single straight line, and that an intersection between an input plane Fin (i.e., one of the end faces of the MMF 83) and the fiber axis Afr coincides with a focal point Zfp of the lens 81.
In the above-described optical transmission system Scv, an optical signal from the lens 81 is focused on the input plane Fin of the MMF 83, and therefore efficiently enters the MMF 83 with small coupling losses. Thereafter, the optical signal suffers increasingly more influence of mode dispersion as it is propagated through the core of the MMF 83. As a result, an optical signal having a plurality of modes associated with different propagation delay amounts goes out at an output plane Fout of the MMF 83 (i.e., the end opposite to the input plane Fin). The optical signal outputted from the MMF 83 enters the mode separator 84, where only the necessary mode(s) is selected. Thereafter, the receiver 85 receives the optical signal which has been subjected to the selection at the mode separator 84. Thus, the receiver 85 is allowed to receive an optical signal with a reduced influence of mode dispersion, whereby the transmission bandwidth of MMF 83 is improved.
However, the mode separator 84, which is essentially an optical system comprising a number of lenses and mirrors, may be expensive. Moreover, the use of such an optical system complicates the overall structure of the optical transmission system Scv. Furthermore, the optical axis alignment between components of the mode separator 84 requires high precision. This presents a problem because it takes considerable cost to construct and maintain the conventional optical transmission system Scv.
There is an additional problem in that it is difficult to improve the mode selection efficiency of the mode separator 84. As used herein, the “mode selection efficiency” is a ratio of the output power to the input power of the mode separator 84 for a given mode. If the mode selection efficiency is poor, the input power for the receiver 85 is diminished, so that it may become necessary to enhance the power of the optical signal originating from the light source 82 and/or the photodetection sensitivity of the receiver 85, or to provide an optical amplifier subsequent to the mode separator 84, leading to increased cost for constructing and maintaining the conventional optical transmission system Scv.